This invention relates to a front derailleur for a bicycle which is located near a multistage front sprocket means comprising at least two sprocket wheels arranged in parallel, each having a different number of teeth and teeth spacing, and which is used for shifting a drive chain to one of the sprocket wheels. The multistage front derailleur is conventionally comprised (FIG. 4) of a base member 1, linkage members 2, 3 and a movable member 4 having a chain guide 5 mounted thereon, each of these members being coupled with pins P1, P2, P3 and P4 to constitute a parallel moving chain guide coupled to a four component linkage mechanism with these pins as supporting points. The linkage mechanism and associated chain guide 5 are so constructed that the chain guide moves in parallel with respect to the bicycle along the axial direction of the multistage front sprocket means 6 during the movement of the movable member 4. When a chain C is shifted by the chain guide 5, an overshifting operation is required beyond a shift quantity which is necessary for shifting the chain C, and a user who is not accustomed to the overshifting operation may not perform speed changes smoothly.
To further describe the problem, when a chain C is shifted from a small-diameter sprocket 61 on the low-speed side of the front sprocket means 6 to a large-diameter sprocket 62 on the high-speed side of the front sprocket means or vise versa, the shifting movement of the chain C by the chain guide 5 is made somewhat larger than the space between the sprockets 61 and 62. In other words, an overshifting operation in which the shifting movement of the chain C by the chain guide 5 is made slightly larger than the distance between the sprockets 61 and 62 is performed to provide a reliable and smooth shifting of the chain C with respect to the sprocket 61 or the sprocket 62. However, since the chain guide 5 moves parallel to the bicycle in the conventional front derailleur, when the parallel movement of the chain guide 5 is set larger than the movement of chain C, it must be returned by the amount of overshift after the chain has been shifted to a given sprocket 61 or 62. When it is not returned by the amount of the overshift, the chain C is pressed in the shift direction by the chain guide 5 and remains biased in the shift direction with respect to the sprocket 62 or the sprocket 61. Therefore, if the amount of shifting movement of the chain C is made larger to improve the speed change performance, a problem occurs in that the chain C is likely to be disengaged from the sprocket 62 or the sprocket 61 unless a restoring operation is performed. Furthermore, without the overshift correction, the chain C always comes into sliding contact with the inner side of a face of the chain guide 5, thus causing problems with respect to the durability of the chain guide and noise which is created by the contact of the chain C with the chain guide.
The present invention is designed to overcome the above noted disadvantages of the conventional front derailleur. Thus, an object of this invention is to provide a front derailleur wherein the chain guided by the chain guide is moved by an amount required for shifting with respect to the multistage front sprocket stage without requiring or introducing overshift during the speed change operation to perform a smooth and reliable shifting from one sprocket to another of the sprocket means. The present invention also reduces the amount of noise generated and the wear on the chain guide.
Briefly, the invention achieves the foregoing objects by providing a front derailleur comprising a base member; two linkage members pivoted on the base member; and, a movable member pivoted to the two linkage members and having a chain guide mounted thereon. All four members are pivotally coupled through pins at the ends of the two linkage members and at the base member. The base member, two linkage members, and the movable member constitute a non-parallel chain guide moving four-connected component linkage mechanism in which a swinging movement imparted to the chain guiding portion of the chain guide is larger than the movement of the moving member connected therewith when the moving member moves with respect to the base member in the axial direction of the sprocket means, whereby the chain is reliably shifted without any overshift.
In a further description of the invention, the chain guide is arranged to have a chain guiding portion swingingly moved around a fixed position on the movable member during the motion of the movable member through an approximately fan-shaped track. When the chain is shifted to one sprocket of the sprocket means, the chain can be guided to the desired sprocket through the swinging motion of the chain guiding portion without any overshift. As a result, the shifting of the chain to the sprocket can be performed reliably and smoothly.
Other objects and novel characteristics of the invention will be apparent from the description of the embodiment in accordance with the accompanying drawings.